Add '///' to Dart comment-token configuration

Fixes #12537

languages.toml

@@ -1671,7 +1671,7 @@ scope = "source.dart"
 file-types = ["dart"]
 roots = ["pubspec.yaml"]
 auto-format = true
-comment-token = "//"
+comment-tokens = ["//", "///"]
 block-comment-tokens = { start = "/*", end = "*/" }
 language-servers = [ "dart" ]
 indent = { tab-width = 2, unit = "  " }